Brother use a system of returning the residual print waste toner from the drum, to the toner supply hopper, via a charge brush which is in contact with the drum and charged differently during the clean cycle of the drum. The charged residual toner is transferred back to the developer roller and dropped back into the supply hopper. This is why most Brother cartridges, unlike most makes, do not have a waste toner hopper in the drum unit.
When the toner is getting low, the electronics
increase the charge on the developer roller to compensate for the mix of
waste and good toner. This process continues until the maximum charge
can no longer be accepted by the toner. This results in the charge brush
failing to operate properly and hence waste toner is not returned to
the toner hopper, but remains on the drum. This causes the backgrounding
which appears on the printed page. Even blocking the low toner sesor
will not prevent this from happening. This is a brief description of a
rather lengthy complicated prcess.